Primary Key vs. Unique

Attribute	Primary Key	Unique
Definition	A primary key is a column or a set of columns that uniquely identifies each row in a table.	A unique constraint ensures that all values in a column are different.
Null Values	Primary key columns cannot contain null values.	Unique columns can contain null values, but only one null value is allowed.
Number of Columns	There can be only one primary key in a table.	There can be multiple unique constraints in a table.
Index	Automatically creates a clustered index in most databases.	Creates a non-clustered index in most databases.

Definition

Primary Key and Unique are both constraints in a relational database that enforce the uniqueness of values in a column or a set of columns. A Primary Key is a column or a set of columns that uniquely identifies each row in a table. It must contain unique values and cannot have NULL values. On the other hand, Unique constraint ensures that all values in a column or a set of columns are unique, but it allows NULL values.

Usage

Primary Key is typically used to uniquely identify each row in a table and is often used as a foreign key in other tables to establish relationships between them. It is a crucial component of database design as it ensures data integrity and helps in maintaining the referential integrity of the database. Unique constraint, on the other hand, is used to enforce the uniqueness of values in a column or a set of columns, but it does not necessarily identify each row uniquely.

Implementation

Primary Key is implemented using the PRIMARY KEY constraint in SQL, which can be applied to one or more columns in a table. It automatically creates a unique index on the specified column(s) and does not allow NULL values. Unique constraint, on the other hand, is implemented using the UNIQUE constraint in SQL, which can also be applied to one or more columns in a table. It creates a unique index on the specified column(s) but allows NULL values.

Performance

Primary Key is often used as the clustered index in a table, which physically orders the rows in the table based on the values of the Primary Key column(s). This can improve the performance of queries that involve searching or sorting based on the Primary Key. Unique constraint, on the other hand, creates a non-clustered index on the specified column(s), which may not provide the same level of performance benefits as a clustered index.

Relationships

Primary Key is used to establish relationships between tables in a database by defining foreign keys that reference the Primary Key of another table. This ensures data integrity and helps in maintaining the consistency of data across related tables. Unique constraint, on the other hand, does not directly establish relationships between tables but can be used to enforce uniqueness in columns that are not Primary Keys.

Constraints

Primary Key constraint enforces both uniqueness and non-nullability of values in the specified column(s), making it a more stringent constraint compared to Unique constraint. This ensures that each row in a table is uniquely identified and that the Primary Key column(s) cannot contain NULL values. Unique constraint, on the other hand, only enforces uniqueness of values but allows NULL values in the specified column(s).